This Project will use genetically-modified mice to study the overall hypothesis that cardiac K plus channels are expressed in a cell-specific fashion, and that this expression has important functional consequences. Two K plus channel genes are proposed for study: minK and Kv1.5, whose expression is thought to result in Iks and Ikur respectively. Program investigators have generated a mouse in which the minK gene has been removed, and replaced by a reporter (lacZ) whose expression is therefore driven by minK regulatory elements. Preliminary studies in the knockout indicate that, unexpectedly, expression of minK is highly restricted in the adult mouse heart, to the conducting system. Specific Aim 1 of this Project will determine the functional consequences of deletion of the minK gene. These studies will test not only the hypothesis that minK expression is required to recapitulate native Iks, but also related hypotheses such as whether expression is similarly restricted in other animals, and whether the electrophysiologic effects of deleting minK extend to altered expression or function of other cardiac ion channels. In the second Specific Aim, DNA regulatory elements responsible for restricted expression of minK to the conducting system will be identified using transgenic approaches. In the third Specific Aim, the gene structure of Kv1.5 will be modified by adding a new pharmacologic signature and an epitope tag. Mice carrying the modified gene will then be generated to test the hypothesis that Kv1.5 expression is similarly regionally determined, and to evaluate the resultant functional consequences. These new approached to studying cellular localization and function of ion channels should provide important new information on the mechanisms underlying the heterogeneous electrophysiologic behavior of the heart, the overall goal of this Program.